Inductive loads switched by a power transistor can produce voltages high enough so that without over-voltage protection, the power transistor may be permanently damaged.
In the past, one way of providing protection for power MOSFETs has been utilizing a feedback path comprising a series connected zener diode and a conventional diode connected across the gate-drain of the MOSFET as shown in FIG. 1.
To protect against over-voltage, the zener voltage Vz plus the diode drop voltage Vd plus the MOSFET gate to source voltage Vgs must be less than the MOSFET breakdown voltage. As a practical matter, the total voltage drop of Vz+Vd+Vgs must be much less than the MOSFET breakdown voltage due to the fact that these elements do not track fabrication process variations in the devices.
It is desirable to provide a power transistor device that has over-voltage protection integral to the device.